javascript - Continuous Color Transition

Question

I'm trying to create a continuous color transition like the one in Windows 8 installation, We're getting your PC ready.

I couldn't figure out how to write the shift function. Checking all R, G, B values and matching the current color to the next color.

Can anyone help me on this? Or let me know if there's a better approach to it than this?

function switchColor(id) {
    var elm = document.getElementById(id);

    //  getting elm's current rgb values
    var elmColor = window.getComputedStyle(elm).getPropertyValue("background");
    var startIndex = elmColor.indexOf("(");
    var finishIndex = elmColor.indexOf(")");
    var elmRGB = elmColor.substring(startIndex + 1, finishIndex);
    var currentColor = elmRGB.split(",");
    for (var i = 0; i<3; i++) { currentColor[i] = currentColor[i].trim(); }

    //  generating a random color => [r, g, ,b]
    var nextColor = [];
    for (var i = 0; i < 3; i++) {
        nextColor[i] = Math.floor(Math.random()*250);
    }

    //  function to convert rgb array to hex color => [r, g, b] = #rgb
    function rgbToHex(clr) {
        var rgb = clr;
        var hex;
        var hex1 = rgb[0].toString(16);
        var hex2 = rgb[1].toString(16);
        var hex3 = rgb[2].toString(16);
        if (hex1.length < 2) { hex1 = "0" + hex1; }
        if (hex2.length < 2) { hex2 = "0" + hex2; }
        if (hex3.length < 2) { hex3 = "0" + hex3; }
        return hex = "#" + hex1 + hex2 + hex3;
    }

    //  checking if nextColor rgb values are greater than current rgb's
    //  so we can increase or decrease for smooth transition
    var status = [];
    for (var i = 0; i < 3; i++) {
        if (nextColor[i] > currentColor[i]) { status.push(1); }
        else { status.push(0); }
    }

    //    this isn't part of the code, just testing
    elm.style.background = rgbToHex(nextColor);

    function shift() {
        //  shift between colors
        //  modify currentColor's rgb values and apply it to the elm
        //  elm.style.background = rgbToHex(currentColor);
    }
    var handler = setInterval(shift, 100);
}
setInterval(function() { switchColor("sandbox"); }, 2000);

JSFiddle

Answer 1

Check this JSFiddle for a transition with a fancy graph.

/* ==================== Required Functions ==================== */
// This is required to get the initial background-color of an element.
// The element might have it's bg-color already set before the transition.
// Transition should continue/start from this color.
// This will be used only once.
function getElementBG(elm) {
var bg= getComputedStyle(elm).backgroundColor;
bg= bg.match(/((.*))/)[1];
bg= bg.split(",");
for (var i = 0; i < bg.length; i++) {
bg[i] = parseInt(bg[i], 10);
}
if (bg.length > 3) { bg.pop(); }
return bg;
}

// A function to generate random numbers.
// Will be needed to generate random RGB value between 0-255.
function random() {
if (arguments.length > 2) {
return 0;
}
switch (arguments.length) {
case 0:
return Math.random();
case 1:
return Math.round(Math.random() * arguments[0]);
case 2:
var min = arguments[0];
var max = arguments[1];
return Math.round(Math.random() * (max - min) + min);
}
}

// Generates a random RGB value.
function generateRGB(min, max) {
var min= min || 0;
var max= min || 255;
var color= [];
for (var i = 0; i < 3; i++) {
var num = random(min, max);
color.push(num);
}
return color;
}

// Calculates the distance between the RGB values.
// We need to know the distance between two colors
// so that we can calculate the increment values for R, G, and B.
function calculateDistance(colorArray1, colorArray2) {
var distance = [];
for (var i = 0; i < colorArray1.length; i++) {
distance.push(Math.abs(colorArray1[i] - colorArray2[i]));
}
return distance;
}

// Calculates the increment values for R, G, and B using distance, fps, and duration.
// This calculation can be made in many different ways.
function calculateIncrement(distanceArray, fps, duration) {
var fps= fps || 30;
var duration= duration || 1;
var increment= [];
for (var i = 0; i < distanceArray.length; i++) {
var incr = Math.abs(Math.floor(distanceArray[i] / (fps * duration)));
if (incr == 0) {
incr = 1;
}
increment.push(incr);
}
return increment;
}

// Converts RGB array [32,64,128] to HEX string #204080
// It's easier to apply HEX color than RGB color.
function rgb2hex(colorArray) {
var color = [];
for (var i = 0; i < colorArray.length; i++) {
var hex = colorArray[i].toString(16);
if (hex.length < 2) { hex = "0" + hex; }
color.push(hex);
}
return "#" + color.join("");
}

/* ==================== Setup ==================== */
// Duration is not what it says. It's a multiplier in the calculateIncrement() function.
// duration = 1-4, fast-to-slow
var fps= 30;
var duration= 3;
var transElement= document.body;
var currentColor= getElementBG(transElement);
var transHandler= null;

startTransition();

/* ==================== Transition Initiator ==================== */
function startTransition() {
clearInterval(transHandler);

targetColor= generateRGB();
distance= calculateDistance(currentColor, targetColor);
increment= calculateIncrement(distance, fps, duration);

transHandler = setInterval(function() {
transition();
}, 1000/fps);
}

/* ==================== Transition Calculator ==================== */
function transition() {
// checking R
if (currentColor[0] > targetColor[0]) {
currentColor[0] -= increment[0];
if (currentColor[0] <= targetColor[0]) {
increment[0] = 0;
}
} else {
currentColor[0] += increment[0];
if (currentColor[0] >= targetColor[0]) {
increment[0] = 0;
}
}

// checking G
if (currentColor[1] > targetColor[1]) {
currentColor[1] -= increment[1];
if (currentColor[1] <= targetColor[1]) {
increment[1] = 0;
}
} else {
currentColor[1] += increment[1];
if (currentColor[1] >= targetColor[1]) {
increment[1] = 0;
}
}

// checking B
if (currentColor[2] > targetColor[2]) {
currentColor[2] -= increment[2];
if (currentColor[2] <= targetColor[2]) {
increment[2] = 0;
}
} else {
currentColor[2] += increment[2];
if (currentColor[2] >= targetColor[2]) {
increment[2] = 0;
}
}

// applying the new modified color
transElement.style.backgroundColor = rgb2hex(currentColor);

// transition ended. start a new one
if (increment[0] == 0 && increment[1] == 0 && increment[2] == 0) {
startTransition();
}
}

body {
  background: white;
}